Mature liver flukes, Fasciola hepatica, of different ages were isolated from the bile ducts of experimentally infected rats. Their energy metabolism was studied during aerobic incubation with [6-14C]glucose. The results showed that the aerobic potentials of the parenchymal liver flukes are not lost immediately after arrival in the bile ducts, ... read more but in a later phase. During the development of the newly excysted juvenile into the mature adult the major part of ATP production in aerobic incubations is successively contributed by three different pathways of glucose breakdown. The Krebs cycle, which is by far the main energy-yielding pathway of the juvenile fluke, is gradually replaced by aerobic acetate formation and, finally, by the anaerobic dismutation reactions of the adult liver fluke. This observed decrease in Krebs-cycle activity per mg protein is not the result of a decrease in activity per individual fluke. The Krebs-cycle activity per fluke actually increases enormously during its whole development. This indicates that the aerobic potential of adult F. hepatica is not just a remnant of earlier aerobic stages but that classical, mammalian type mitochondria are produced during the entire development of the fluke. Calculations are presented which demonstrate that the Krebs-cycle activity of the developing F. hepatica is directly proportional to the surface area of the fluke. This supports our view that Krebs-cycle activity is limited by the diffusion of oxygen and can only occur in the outer layer of the liver fluke during its entire development in the final host. show less